1. Field of the Invention
The present invention relates to nuclear gauges for making measurements of traveling webs in continuous sheet-making systems.
2. State of the art
In continuous sheet-making systems, on-line measurements are highly desirable. The on-line measurements can provide, for instance, early indications of upsets in process conditions, thus allowing process controls to be effected before substantial quantities of substandard material are produced. In practice, however, accurate on-line measurements are difficult to make. The measurement difficulties arise, in part, because modern sheet-making machines are large and operate at high speeds. Some paper-making machines, for example, produce paper webs that are 100 to 400 inches wide at rates of up to 100 feet per second.
For making on-line measurements of properties of traveling webs in continuous sheet-making systems, it is common to employ sensors that periodically traverse, or scan, the webs. One type of scanning sensor is the nuclear gauge. In operation, nuclear gauges direct nuclear radiation (beta rays) against a surface of a traveling web while detecting the absorbed (or transmitted) radiation. (The quantity of nuclear radiation absorbed over a given area is a measure of the basis weight of the absorbing material.) Nuclear scanning gauges typically use radioactive krypton gas as the beta-ray source.
When using nuclear gauges, safety is a major concern. For safety reasons, it is important that nuclear sources are appropriately shielded, especially when not in use, to prevent accidental exposure of personnel who might be working near the gauge. In conventional practice, shielding of a nuclear gauge is accomplished by mounting the nuclear source material to a protective housing that has a shuttered window. When the nuclear gauge is in use, the shuttered window is opened to allow radiation to be emitted onto a traveling web. When the gauge is not in use, the shuttered window is closed, thus blocking radiation and reducing the opportunities for accidental exposure.
Although shielding of nuclear gauges is necessary, the design of the shielding must not impair the accuracy of measurements that are made by the gauge.